Clocking circuits are used for a variety of purposes in digital circuits on both board level systems and integrated circuit (IC) devices. For example, global clock signals are used to synchronize various circuits across the board or IC device. Many complex digital systems, such microprocessors and field programmable gate arrays (FPGAs), have multiple clock signals at different frequencies. For example, in some microprocessors, internal circuits are clocked by a first clock signal at a first clock frequency while input/output (I/O) circuits are clocked by a second clock signal at a second, different clock frequency.
Multiple clock generating circuits can be used to generate the multiple clock signals. However, most systems use one clock generating circuit to derive other clock signals from the first clock signal. For example, clock dividers are used to generate one or more clock signals of lower clock frequencies from a reference clock. Typically, clock dividers divide the frequency of the reference clock signal by an integer value.